Data from: Competitor relatedness, indirect soil effects and plant co-existence

How species co-exist is a central question in ecology. Niche differentiation can prevent two species from excluding each other by competition. However, this interaction can vary in space because of internal factors such as intraspecific genetic variation, or external ones such as the presence of a third species. While these effects have been repeatedly observed, their joint effect on the outcome of competition has rarely been considered. We experimentally investigated how a dominant ecosystem engineer (the shrub Thymus vulgaris) affects interactions between two co-occurring and closely related annuals (Medicago minima and M. rigidula). We ask first if the outcome of their interaction depends on the genetic identity of the competitors, and in particular if intraspecific interaction differs between genetically related versus unrelated conspecifics. Second, we ask if co-existence of the two species is modified by the presence of thyme plants altering local soil conditions. We grew genotypes collected in nature of both annual species in inter- and intraspecific competition experiments using soil collected directly underneath thyme plant (thyme soil), and soil collected away from thyme plants (no-thyme soil). We found that thyme soil affected interactions in a way that shifted the species’ rank. Crucial for this result was the division of intraspecific competition between genetically related and unrelated individuals. Intraspecific competition was significantly less detrimental to kin than to unrelated conspecifics. In highly structured plant populations, such effects could favor local dominance of a patch by one species. However this kin effect occurred for each species only in its preferred soil (thyme versus no-thyme), promoting a stabilizing co-existence mechanism at the landscape level. Synthesis: This study shows that the importance of intraspecific competition relative to interspecific competition can be highly dependent on the genotype identity of intraspecific competitors and of the local environment in which the interaction occurs. The local environment can itself be modified by the presence of a third species in the community. These results emphasize how, in order to understand the overall problem of species co-existence, it can be insightful to divide this into smaller local scale problems of co-existence.物种共存机制是生态学领域的核心科学问题之一。生态位分化（niche differentiation）可通过种间竞争作用避免两个物种发生相互排除。然而，这类种间相互作用会因内部或外部因素在空间上产生变异：内部因素包括种内遗传变异（intraspecific genetic variation），外部因素则如第三物种的存在。尽管这类效应已被多次观测到，但它们对竞争结局的联合影响却极少被纳入考量。本研究通过实验探究了优势生态系统工程师（ecosystem engineer）——普通百里香（Thymus vulgaris）对两种同域分布且亲缘关系相近的一年生植物：小苜蓿（Medicago minima）与硬荚苜蓿（Medicago rigidula）之间的种间相互作用的调控效应。我们首先探讨两个核心问题：其一，两种植物的竞争结局是否取决于竞争者的遗传身份？具体而言，亲缘关系相近的同种个体与无亲缘关系的同种个体之间的种内竞争（intraspecific competition）是否存在显著差异？其二，百里香植株通过改变局域土壤条件，是否会改变两种植物的共存格局？我们分别采集两种一年生植物的自然种群基因型，以百里香植株根际土壤（百里香土壤）与远离百里香植株的土壤（非百里香土壤）为栽培基质，开展种间与种内竞争实验。研究发现，百里香土壤会改变两种植物的竞争等级排序，进而调控其相互作用。该结果的关键前提是将种内竞争划分为亲缘个体间与非亲缘个体间的竞争：相较于非亲缘同种个体，亲缘个体间的种内竞争带来的适合度损失显著更低。在高度结构化的植物种群中，这类效应可促使单一物种在局部斑块占据优势地位。但值得注意的是，这种亲缘效应仅在对应物种偏好的土壤类型中显现：小苜蓿偏好非百里香土壤，硬荚苜蓿偏好百里香土壤，从而在景观尺度上形成稳定的共存机制。综合分析表明：种内竞争相较于种间竞争的重要性，高度依赖于种内竞争者的遗传身份以及相互作用发生的局域环境；而局域环境本身又可因群落中第三物种的存在而发生改变。本研究结果强调，若要全面理解物种共存的整体机制，将其拆解为更小尺度的局域共存问题进行分析，可获得更具启发性的科学认知。